Welcome from California.
It sounds like you need to whittle out a custom forearm that works with however you have to position your bad arm.
I have kind of a similar problem. I suffered some damage to my left shoulder few years back.
I have a Mossberg 500, with big plastic forestock having a built-in flashlight. My left hand is not big and strong enough to hold on to the thing.
I've done lots of physical therapy on my arm but I've come to the conclusion that I need a custom foregrip on that shotgun.
When the mag tube is loaded and the gun is heavy I can hang on to it, but by that last shot it's so light it just bucks up out of my hand every time.
A strap would help, but I feel they're kind of awkward for home-defense situations. In a real emergency the strap gets in your way.
Anyhow I'm going to try and mock up something makes it easier for me to keep a grip on this machine. Either that or I'm going to ditch the flashlight forearm completely and put a regular forearm with an accessory flashlight.